1. Field of the Invention
The invention relates generally to methods of detecting organic material, and more particularly to a method of detecting organic material using terahertz spectroscopy.
2. Description of the Related Art
Spectroscopy over the terahertz (THz) region may be divided into two basic techniques: time-domain and frequency-domain. Time domain systems employ optical pulses from a mode locked laser to produce THz pulses through a demodulation process in a photo-conductive (i.e., “Auston”) switch (PCS). The THz pulse is passed through the sample of interest before being focused onto a second PCS (the detector) that is driven by a delayed optical pulse from the same mode locked laser. The delay of this optical detector pulse is varied; and the detector PCS photocurrent is measured as a function of delay to obtain a THz autocorrelation function. A normalization and Fourier transformation are applied to this autocorrelation to produce a frequency-dependent transmission through the sample of interest. As in Fourier Transform spectroscopy, the spectral resolution is determined primarily by the length of the delay line, which is very difficult to increase much past 1 cm, rendering a typical resolution of 1 cm−1 or 30 GHz.
In the frequency-domain technique, on the other hand, continuous-wave THz radiation is produced through photomixing of the combined output of two single-frequency lasers in a PCS. The wavelength of one (or both) of the lasers is tuned to vary the THz output frequency. In most spectroscopic applications of photomixing, the THz output beam from the PCS is coupled to a sensitive broadband thermal detector (e.g., LHe bolometer or Golay cell), making the overall signal processing incoherent and phase insensitive. Coherent (homodyne) detection can be achieved at room temperature by mixing the same optical radiation from the lasers in a detector PCS onto which the THz signal is also incident. This provides greater sensitivity and faster data acquisition than the incoherent technique, and preserves phase information.
Some of the benefits of the coherent frequency-domain technique compared to the time-domain technique are no moving parts, higher frequency resolution, the ability to selectively scan specific frequency regions of interest with adjustable resolution, the potential to highly integrate the inexpensive semi-conductor laser chips into an extremely small form factor package and low current requirements that may allow battery operation. Also, unlike pulsed systems, CW photomixing results in all of the THz power being concentrated at a single THz frequency, thus improving spectral density and signal-to-noise ratio at that frequency.
Despite the advantages, historically, it has been difficult to realize compact frequency-domain spectrometers due to the challenges associated with the construction and control of the dual lasers, namely mode-matching and co-collimation of the two laser beams and precise control of their difference frequency.